The present invention relates to an intensified charge-coupled image sensor and, particularly, to the charge-coupled device (CCD) which is utilized therein.
An intensified charge-coupled image sensor 10, shown in FIG. 1, comprises an image intensifier section having a photoemissive cathode (not shown) on an interior surface of an input faceplate and a header assembly having a charge-coupled device (CCD) 12 located at the focal plane of the image sensor. Such a structure is described in U.S. Pat. No. 4,355,229, issued to H. S. Zimmerman et al. on Oct. 19, 1982, and assigned to the assignee of the present invention. The Zimmerman et al. patent is incorporated by reference herein for the purpose of disclosure.
The operation of the intensified charge-coupled image sensor 10 is well understood. During the so-called integration time, a scene is projected onto the photoemissive cathode on the interior surface of the input faceplate, and photoelectrons are released therefrom in a pattern corresponding to the intensity of the radiation incident on the cathode. The photoelectrons impinge upon the A register of the CCD 12 shown in FIG. 2.
Upon the completion of the integration time (e.g., during the vertical blanking interval of commercial television), the charge signals which have accumulated (a "field") are transferred, in parallel, in the column direction from the A to the B register by the application of the multiple phase voltages .phi..sub.A1 . . . .phi..sub.A3 and .phi..sub.B1 . . . .phi..sub.B3. The charges subsequently are transferred a row at a time, from the B register to the C register, and after each row of charges reaches the C register, it is serially shifted out of the C register in response to the shift voltages .phi..sub.C1 . . . .phi..sub.C3. The transfer of charges from the B to the C register occurs during a relatively short time (the horizontal blanking time of commercial television, which is about 10 .mu.s) and the serial shifting of the C register occurs at relatively high speed (during the horziontal line display time of commercial television). During the transfer of a field from the B to the C register, a new field may be integrated in the A register.
When the CCD 12 is used in an intensified charge-coupled image sensor 10, it is necessary to thin the substrate, at least in the A register to a thickness of about 10.mu. (microns) to minimize lateral dispersion of the charge produced by the incident photoelectrons. The B and C registers are masked to prevent photoelectrons from impinging thereon.
In one practical approach to thinning substrates, the number of CCD's 12 produced on a wafer is relatively small. First, using a two-inch silicon wafer, up to three such devices, each about 12.7 mm.times.20.3 mm (0.5 inches.times.0.8 inches), as shown in FIG. 3, are fabricated at the same time on a common substrate, employing photolithographic techniques. Then, two of the three devices are masked, that is, except for the back surface of the device being thinned, the entire wafer is coated with a substance (a "resist") which is not attached by the chemical bath (an acid) used to thin the substrate. Then, the entire wafer is immersed in the thinning bath, and the wafer is spun about an axis passing through the center of the device being thinned. After the desired amount of thinning of the device is obtained, the wafer is removed from the bath, the resist is removed, and, then, the masking and other processing steps are repeated for each additional device. Then, the wafer is cut apart in such a way that there is a thick substrate border surrounding the thin A register of each device. This thick border, shown in FIG. 4, provides some stiffness and mechanical support for the relatively fragile thinned substrate region of the A register.
While the process above has resulted in the production of many operational CCD's 12, it is not without its problems. One is that it is difficult to obtain uniform thinning throughout the entire imaging portion of the A register of the device. It is thought that because of the rectangular shape of the device, the acid bath sometimes does not attack as strongly some of the edge and corner regions of the device as the center of the device, and these edges and corners, therefore, are sometimes thicker in the final product than the center region of the device. Such non-uniformity is undesirable as it sometimes causes non-uniformities in the picture information produced by the imager. Also, as a practical matter, one cannot manufacture at the same time a large number of devices on the same wafer, even a large wafer. If one were to employ a larger wafer, say 4" or 5" in diameter, there would be problems in spinning the wafer during thinning about an axis considerably displaced from the center of the wafer, and, therefore, it would be difficult to utilize the outer edge portion of the wafer (recall that the axis about which the spinning takes place passes through the center of the region being thinned). In addition, the yield using this method is not as high as desired. Also, because of the fragility of the thinned substrate, it is very difficult to test the devices after they are thinned. The reason is that the test probes tend to cause the thinned substrate to become broken or otherwise damaged.
U.S. Pat. No. 4,266,334, issued to T. W. Edwards et al. on May 12, 1981, and incorporated by reference herein for the purpose of disclosure, describes a method of manufacturing a large number of thinned substrate CCD's on a single semiconductor wafer. Instead of thinning the A registers of the individual CCD's one at a time, as in the previous process described above, the entire wafer is thinned in a rotary etch bath to the desired thickness over its entire center area, leaving only an unthinned rim around the peripheral edge of the wafer for support. Then, a glass plate is laminated or glued onto the thinned surface of the wafer to add structural support during testing and removal of the individual CCD's from the large wafer. If the CCD's are to be used as "images", i.e., solid state devices in which photons are imaged directly on the CCD, then it is not necessary to remove the glass plate. The glass support plate must, however, be delaminated or otherwise removed from the wafer before the CCD's contained therein can be used in an intensified charge-coupled image sensor. The glue used to affix the glass plate to the CCD is incompatible with the formation of the photoemissive cathode formed on the interior surface of the input window of the image intensifier section of the image sensor, and the glass window also attenuates the photoelectrons from the cathode, thereby preventing them from impinging upon the A register of the CCD.
U.S. Pat. No. 4,465,549, issued to I. G. Ritzman on Aug. 14, 1984, and assigned to the assignee of the present invention, discloses a method of removing the glass support plate from a thinned wafer containing a large number of CCD's. The wafer is subsequently sectioned to provide the individual CCD's 12. As shown in FIG. 5, each of the thinned CCD's 12 has a substantially uniform thickness of about 10.mu. and is too delicate to handle without damage. Thus, copending U.S. patent application Ser. No. 494,288, filed by J. A. Zollman et al. on May 13, 1983, discloses a support frame (not shown) which is brazed to one surface of a thinned CCD to provide the required rigidity for handling and mounting the CCD 12 within the intensified charge-coupled image sensor 10. In prior art intensified charge-coupled image sensors 10, the CCD 12 with the attached frame is brazed to an insulative header. The initial brazing of the frame to the CCD and the subsequent brazing of the frame with the attached CCD to the insulative header has resulted occasionally in scrape due to the flow of braze material onto the CCD and to misalignment of the CCD on the insulative header.
It has been determined that the overall yield of CCD's for use in intensified charge-coupled image sensors and produced by the above-described methods must be increased in order to lower the cost of the intensified charge-coupled image sensors. More specifically, the fabrication of the CCD must be tailored to the requirements of the intensified charge-coupled image sensor, and those processing steps which are not compatible with the subsequent processing and operation of the intensified charge-coupled image sensor must be eliminated. Furthermore, it is desirable to provide a CCD, which is uniformly thin in the active register areas, i.e., in the A, B and C registers, yet has a sufficient structural integrity, thereby obviating the need for brazing a support frame to the periphery of the device.